It is commonly the case that in order to reduce power consumption in radio transceivers, signal output power is changed according to the communication distance. There are techniques that changes the number of active high-output amplifiers provided in the last stage of a transceiver according to signal output power to allow the radio transceiver to operate with the optimum power efficiency at all times.
An amplifier circuit using a transformer is disclosed in “Fully Integrated CMOS Power Amplifier With Efficiency Enhancement at Power Back-Off”, G, Liu, P. Haldi, T.-J. K. Liu, and A. M. Niknejad, IEEE Journal of Solid-State Circuits, Vol. 43, No. 3, pp. 600-609, March 2008. Japanese Laid-Open Patent Publication No. 9-307367 discloses a power combining method using multiple bridge-type single-ended push-pull power amplifiers which amplify an input RF input signal and output the amplified RF input signal to a common combining transformer, in which no bias voltage is supplied to transistors constituting a bridge-type single-ended push-pull circuit to which the RF signal is not input.